Apparatus and method for electrostatic coating of an abluminal stent surface

ABSTRACT

A stent mandrel fixture for supporting a stent during the electrostatic application of a coating substance is provided.

TECHNICAL FIELD

This invention relates to apparatus and method for electrostatic coatingof stents, more specifically to a stent mandrel fixture used during theelectrostatic coating process.

BACKGROUND

Blood vessel occlusions are commonly treated by mechanically enhancingblood flow in the affected vessels, such as by employing a stent. Stentsact as scaffoldings, functioning to physically hold open and, ifdesired, to expand the wall of affected vessels. Typically stents arecapable of being compressed, so that they can be inserted through smalllumens via catheters, and then expanded to a larger diameter once theyare at the desired location. Examples in the patent literaturedisclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S.Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062issued to Wiktor.

Stents are used not only for mechanical intervention but also asvehicles for providing biological therapy. Biological therapy can beachieved by medicating the stents. Medicated stents provide for thelocal administration of a therapeutic substance at the diseased site.Local delivery of a therapeutic substance is a preferred method oftreatment because the substance is concentrated at a specific site andthus smaller total levels of medication can be administered incomparison to systemic dosages that often produce adverse or even toxicside effects for the patient.

One method of medicating a stent involves the use of a polymeric carriercoated onto the surface of the stent. A composition including a solvent,a polymer dissolved in the solvent, and a therapeutic substancedispersed in the blend can be applied to the stent by immersing thestent in the composition or by spraying the composition onto the stent.The solvent is allowed to evaporate, leaving on the surfaces a coatingof the polymer and the therapeutic substance impregnated in the polymer.

The dipping or spraying of the composition onto the stent can result ina complete coverage of all stent surfaces, i.e., both luminal (inner)and abluminal (outer) surfaces, with a coating. However, from atherapeutic standpoint, drugs need only be released from the abluminalstent surface, and possibly the sidewalls. Moreover, having a coating onthe luminal surface of the stent can have a detrimental impact on thestent's deliverability as well as the coating's mechanical integrity. Apolymeric coating can increase the coefficient of friction between thestent and the delivery balloon. Additionally, some polymers have a“sticky” or “tacky” nature. If the polymeric material either increasesthe coefficient of friction or adherers to the catheter balloon, theeffective release of the stent from the balloon upon deflation can becompromised. Severe coating damage at the luminal side of the stent mayoccur post-deployment, which can result in a thrombogenic surface.Accordingly, there is a need to eliminate or minimize the amount ofcoating that is applied to the inner surface of the stent. Reducing oreliminating the polymer from the stent luminal surface also means areduction in total polymer load, which will minimize the material-vesselinteraction and is therefore a desirable goal for optimizing long-termbiocompatibility of the device.

A method for preventing the composition from being applied to the innersurface of the stent is by placing the stent over a mandrel thatfittingly mates within the inner diameter of the stent. A tubing can beinserted within the stent such that the outer surface of the tubing isin contact with the inner surface of the stent. With the use of suchmandrels, some incidental composition can seep into the gaps or spacesbetween the surfaces of the mandrel and the stent, especially if thecoating composition includes high surface tension (or low wettability)solvents. Moreover, a tubular mandrel that makes contact with the innersurface of the stent can cause coating defects. A high degree of surfacecontact between the stent and the supporting apparatus can provideregions in which the liquid composition can flow, wick, and/or collectas the composition is applied to the stent. As the solvent evaporates,the excess composition hardens to form excess coating at and around thecontact points between the stent and the support apparatus, which mayprevent removal of the stent from the supporting apparatus. Further,upon removal of the coated stent from the support apparatus, the excesscoating may stick to the apparatus, thereby removing some of the coatingfrom the stent and leaving bare areas. In some situations, the excesscoating may stick to the stent, thereby leaving excess coatingcomposition as clumps or pools on the struts or webbing between thestruts. Accordingly, there is a tradeoff when the inner surface of thestent is masked in that coating defects such as webbing, pools and/orclumps can be formed on the stent.

In addition to the above mentioned drawbacks, other disadvantagesassociated with dip and spray coating methods include lack of uniformityof the produced coating as well as product waste. The intricate geometryof the stent presents a great degree of challenges for applying acoating material on a stent. Dip coating application tends to provideuneven coatings and droplet agglomeration caused by spray atomizationprocess can produce uneven thickness profiles. Moreover, a very lowpercentage of the coating solution that is sprayed to coat the stent isactually deposited on the surfaces of the device. A majority of thesprayed solution is wasted in both application methods.

To achieve better coating uniformity and less waste, electrostaticcoating deposition has been proposed. Examples in patent literaturecovering electrostatic deposition include U.S. Pat. Nos. 5,824,049 and6,096,070. Briefly, referring to FIG. 1, for electro-deposition orelectrostatic spraying, a stent 100 is grounded and gas is used toatomize the coating solution into droplets 110 as the coating solutionis discharged out from a nozzle 120. The droplets 110 are thenelectrically charged by passing through an electrical field created by aring electrode 130 which is in electrical communication with a voltagesource 140. The charged particles are attracted to the grounded metallicstent. An alternative design for coating a stent with an electricallycharged solution is disclosed by U.S. Pat. No. 6,669,980. U.S. Pat. No.6,669,980 teaches a chamber that that contains a coating formulationthat is connected to a nozzle apparatus. The coating formulation in thechamber is electrically charged. Droplets of electrically chargedcoating formulation are created and dispensed through the nozzle and aredeposited on the grounded stent. Stents coated with electrostatictechnique have many advantages over dipping and spraying methodology,including, but not limited to, improved transfer efficiency (reductionof drug and/or polymer waste), high drug recovery on the stent due toelimination of re-bounce of the coating solution off of the stent, andbetter coating uniformity, and a faster coating process. Formation of acoating layer on the inner surface of the stent is not, however,eliminated with the used of electrostatic deposition. With the use ofmandrels that ground the stent and provide for a tight fit between thestent and the mandrel, formation of coating defects such as webbing,pooling and clumping remain a problem. If a space is provided betweenthe mandrel and the stent, such that there is only minimal contact toground the stent, the spraying can still penetrated into the gapsbetween the stent struts and coat the inner surface of the stent.Conventional stent geometry does not provide for a good Faraday cage dueto the interspace between the struts of the stent. As illustrated byFIG. 2, electric field lines can penetrated into the opening between thestruts and deposit a coating on the inner surface of the stent. This isknown as the “wrap around” effect. Charged particles are not onlydisposed on the outer surface of the stent, but also are wrapped aroundeach strut and are attracted to the inner surface of the stent.

Accordingly, what is needed is an apparatus and method that allows forelectro-deposition or electrostatic spraying of a stent whileeliminating or minimizing the wrap around effect.

SUMMARY

In accordance with one embodiment of the invention a stent mandrelfixture to support a stent during application of a charged coatingsubstance to the stent is provided, comprising a first mandrel componentin conductive contact with the stent and a second mandrel componentpositioned at least partially within a bore of the stent, the secondmandrel component being made from a nonconductive material, being coatedwith a nonconductive material or having a nonconductive sleeve disposedthereon. The first mandrel component can provide for a chargedifferential to the stent relative to the coating substance. Thenonconductive material or the sleeve is capable of collecting a chargeof the same polarity as the coating substance. In some embodiments, thecoating substance includes a conductive solvent.

In accordance with another embodiment, a fixture to support a stentduring application of a charged coating substance to the stent isprovided, comprising a mandrel component extending at least partiallythrough a longitudinal bore of the stent, the mandrel component beingconfigured to minimize or eliminate the wrap around effect of thecharged coating substance around the stent struts to prevent or reducethe amount of coating substance applied to an inner side of the stent.The mandrel component includes an element configured to collect chargedparticles applied to the component. The mandrel component includes anelement configured to repel charged particles applied to the component.The fixture can also include a second mandrel component in electricalcommunication with the stent. The second mandrel component is forapplying a charge to the stent and/or to ground the stent.

In accordance with another embodiment, an electrostatic spray coatingapparatus for electrostatic application of a substance to a stent isprovided comprising a stent mandrel fixture having a first mandrelcomponent in conductive contact with the stent, a second mandrelcomponent positioned at least partially within a bore of the stent, thesecond mandrel component configured to repel charged particles so as toeliminate any, or reduce the amount of, coating substance applied to aninner side of the stent, a coating substance dispenser positioned at adistance away from the stent, and a power source to charge the coatingsubstance.

In accordance with other embodiments, methods of coating a stent with asubstance are provided. The method can include supporting a stent on amandrel, charging the coating substance, applying the coating substanceto the stent, and applying a charge to the stent and/or grounding thestent via the first mandrel component. In one embodiment, the methodcomprising applying charged particles to a part of the mandrel prior theapplying the coating substance to the stent.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a block diagram illustrating an electrostatic spray coatingsystem;

FIG. 2 illustrates the wrap around effect on a stent strut;

FIG. 3 is a block diagram illustrating an electrostatic spray coatingsystem;

FIGS. 4A, 4B and 4C illustrate a mandrel in accordance with oneembodiment of the invention;

FIGS. 5A, 5B and 5C illustrate the mandrel in accordance with variousother embodiments of the invention;

FIG. 6 is chart illustrating spray regimes as a function of appliedvoltage and electrode separation;

FIG. 7 is a magnified cross section of a stent strut with no wrap aroundeffect; and

FIG. 8 is a flowchart illustrating a method of electrostatic spraycoating.

DETAILED DESCRIPTION

The following description is provided to enable any person havingordinary skill in the art to make and use the invention, and is providedin the context of a particular application and its requirements. Variousmodifications to the embodiments will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles, features and teachingsdisclosed herein.

It is believed that the embodiments of the invention can provide for auniform coating, prevent excess waste associated with conventional dipand spray coating processes, and prevent a coating from being formed onthe inner surface of the stent or reduce the amount of coating that isformed on the inner surface of the stent. This reduces the total polymerload on a stent, thereby improving long-term biocompatibility andensuring that most of the coating is on the abluminal surface where itprovides the most benefit. Further, problematic interactions between adelivery mechanism (e.g., delivery balloon) and the stent luminalsurface are eradicated, thereby increasing the ease of stentdeliverability.

FIG. 3 illustrates an electrostatic spray coating system 200. The system200 includes a syringe pump controller 220 communicatively coupled to apump 210 (e.g., a Harvard syringe pump model 11) that pumps a syringe215 holding a composition. As discussed further below, the compositioncan include any type of a coating material such as solvent(s),polymer(s), therapeutic substance(s) or any one or combination of these.The syringe 215 dispenses the composition onto the stent 100 via ametallic dispensing tip, hypotube 225 or other dispenser that is coupledto the syringe 215.

The stent 100 is mounted on a stent mandrel fixture 240 that can providetranslational and rotational movement of the stent 100 during a coatingprocess. The stent 100 can be located, for example, approximately 20-25mm downstream from the hypotube 225. As illustrated by FIGS. 4A, 4B and4C, the mandrel fixture 240 includes a mandrel arm 242 that can extendpartially or all the way through the bore of the stent 100. The mandrelarm 242 includes a larger first diameter section 242A extending to asmaller second diameter section 242B. The larger diameter section 242Ashould be of sufficient size to allow one end of the stent 100 to becrimped thereon or to allow for a friction fit within the one end of thestent 100. The fitting should be tight enough that the stent 100 issupported over the smaller diameter section 242B without making contactwith section 242B. In other words, a gap is disposed between the outersurface of the smaller diameter section 242B and the inner surface ofthe stent 100. The larger diameter section 242B should be made from aconductive material, such as a metal, so as to allow for chargetransmission from the stent 100. In one embodiment, the smaller diametersection 242 can be made from a non-conductive material such as a rubber,plastic, polymer or ceramic material. Alternative, the smaller diametersection 242B can also be made of a conductive material or other materialand insulated with a sleeve 235 made from a non-conductive material(also referred to interchangeably as an insulating material) such asrubber, plastic, polymer or ceramic material. Particular examplesinclude pellethane, nylon, Teflon, polyvinylchloride (PVC), etc.Non-conductive, insulator or insulating refers to the ability of amaterial to prevent the flow of electric current between or amongpoints. Insulation resistance can be measured in megaohms per statedvolume or area. In some embodiments, insulator, insulating ornon-conductive means less or significantly less conductive that thesegment of the mandrel fixture 240 that grounds and/or applies a chargeto the stent, so long as the insulating or non-conductive component(s)of the fixture 240 reduces or prevents the wrap around effect foreliminating or minimizing the formation of the coating on the innersurface of a stent.

In yet another embodiment, smaller diameter section 242B can be coatedwith an insulating material. As used herein, insulating sleeve andcoating, although very different in form, will be used interchangeablyfor brevity. The same concepts that are disclosed with the sleeve applyequally with the use of a coating. With the use of the sleeve 235, thesmaller diameter section 242B must of sufficiently small diameter so asto allow for the gap to exist between the sleeve 235 and the innersurface of the stent 100. The insulating sleeve 235 can have a lengthequal to at least about the length of the stent 100. Shorter lengthsleeves can be used to provide for the wrap around effect at an end ofthe stent. This may be suitable if it is desired to provide for moredrug at one end of the stent. The sleeve 235 can be in tubular form. Thesleeve 235 can also be patterned so as to provide some areas where theconductive mandrel 240 is exposed so as to selectively be able to coatdesignated areas of the inner surface of the stent 100. This may bedesirable in order to provide some degree of friction or adhesivenessbetween a balloon and the stent. In one embodiment of the invention, thesleeve 235 has an inner diameter of about 0.042 inches and an outerdiameter of about 0.054 inches. The thickness of the sleeve 235 dependson the material used. The gap or spacing between the luminal surface ofthe stent 100 and the outer diameter of the sleeve 235 (or the outersurface of a nonconductive smaller diameter section 252B or a coating)can be about 0.005 inches, for example.

Referring to FIG. 5A, in accordance with another embodiment, a mandrelfixture 300 a can include a support member 310 a that engages or isdisposed in one end of the stent 100 and a lock member 320 a thatengages or is disposed in the opposing end of the stent 100. The supportmember 310 a and the lock member 320 a can be coupled together by amandrel arm 330 that extends through the longitudinal bore of the stent100. The arm 330 can be permanently coupled to the support member 310 aand releasable coupled to the lock member 320 a, such as by a screw fitor a friction fit. The support member 310 a and/or lock member 320 a canbe in conductive communication with the stent 100. In one embodiment,the mandrel arm 330 can be made from a non-conductive material oralternatively the sleeve 235 can be disposed over the arm 330. Again, acoating can be used in lieu of the sleeve 235. In some embodiments, oneof the support member 310 a and the lock member 320 a can also be madefrom a non-conductive material, have an insulating sleeve disposedthereon or be coating with an insulating material.

With the use of the mandrel fixtures of FIGS. 4A, 4B, 4C, it is possiblethat some coating defects can be formed in the areas of the end ringsegment or segments of the stent 100 that are disposed over that themandrel section 242A. The coating composition can be equally attractedto the mandrel and form areas of coating conglomeration between thestent struts that are positioned over section 242A. This would beequally true for the embodiment of FIG. 5A as there are now overlappingareas at both ends of the stent 100. To minimize coating defects, thestent 100 can be locked between two surfaces as illustrated in themandrel fixture 300 b of FIG. 5B. The support member 310 b and the lockmember 320 b of FIG. 5B provide ends that are larger than the diameterof the stent 100—as positioned on the fixture—so as to allow the stent100 to be pinched there between. In some embodiments, the stent 100 canbe threaded over the arm 330 and placed against the support member 310b. The lock member 320 b is then screwed or friction fitted onto the arm330 and moved incrementally closer to the support member 310 b so as togently pinch the stent 100 there between. A certain degree of manualadjustment may be necessary to center the stent 100. The support member310 b and/or the lock member 320 b can be made from a conductivematerial so as to ground the stent 100. If only one of the members 310 bor 320 b is in electrical communication with the stent 100, the othermember as well as the arm 300 can be made from a non-conductive materialor can be insulated with a sleeve or a coating.

In another embodiment of the invention, a mandrel fixture 300 c, asshown in FIG. 5C, includes a support member 310 c and/or a lock member320 c that can have a coning end portion 340 that penetrates partiallyinto the stent 100 ends and allows the stent 100 to rest thereon. Withthis embodiment, the necessity of manually centering the stent 100 iseliminated. In one embodiment, at least one of the coning end sections340 can be made from a conductive material. The other end 340 as well asthe arm 330 can be made from a non-conductive material, or can beinsulated with a sleeve or a coating. In some embodiments, a segment ofa tip of the conductive coning end or ends 340 that is disposed withinthe stent 100 can be made from a nonconductive material or can beinsulated by a sleeve or a coating. In some embodiments, the tip 340should be large enough so as to allow for nominal conductive contactbetween the fixture 300 c and the stent 100. The wrap around effect,therefore is further reduced at the stent 100 ends.

In some embodiments, the non-conductive or insulated segment of themandrel fixture 240 should expand across and beyond the length of thestent or a majority of the length of the stent 100 so as to eliminate orminimize any coating from being formed on the inner surface of the stent100. However, as discussed above, the coverage of the non-conductive orinsulating segment of the mandrel fixture 240 can be adjusted so as toallow for some coating to be formed on the inner surface of the stent100. In some embodiments the total length of the inner surface that isprotected from the wrap around effect can be greater than 99%, 95%, 90%,80%, 70%, 60% or 50%.

Referring back to FIG. 3, a power source 245 is coupled to a highvoltage transformer 250 that converts voltage from the power source 245to a high voltage (e.g., up to 20 kV), which is then applied to thehypotube 225. The high voltage ionizes the composition into atomizedionized (e.g., negatively or positively charged) droplets in a spray 230without the need for atomizing air. However, in an embodiment of theinvention, the hypotube 225 may be replaced by an air-assistedatomization nozzle, which connects to an atomizing air source to provideatomizing air to assist in atomization of the composition. Theatomization can be external or internal.

Atomization of the composition using electrospray is governed by theRayleigh Limit, which holds that droplets can be broken apart if thecharge of the droplets exceeds the surface tension of the droplets.Specifically, the maximum charge of a droplet is related to the surfacetension as follows:Q=8π√{square root over (ε_(o) γR ³)}wherein ε_(o) is the permittivity of a vacuum or medium in which thedroplets are atomized; γ is the surface tension of the liquid thedroplet is made of; and R is radius of the droplet. Accordingly, asufficiently high voltage must be applied to the composition to impart ahigh enough charge to overcome the surface tension.

The stent 100 is in electrical contact with the stent mandrel fixture240 at point 241 (for FIG. 4, at an end of the §tent 100) such that anycharge applied to the stent 100 is conducted to the mandrel 240. Thestent mandrel fixture 240 may be grounded and/or electrically connectedto a power source 245 via one end of the mandrel 242, as will bediscussed further below, to form a complete circuit. Therefore, thestent 100 does not collect charge having the same polarity as theatomized ionized droplets of the spray 230. The insulating sleeve 235(or the coating or the non-conductive segment of the mandrel fixture240) is an insulator and therefore does not conduct charge. Any chargedispensed onto the sleeve 235 (e.g., from the spray 230) will thereforebuild up and not be transmitted.

During a coating process, the hypotube 225 dispenses the composition asan ionized and atomized spray 230 onto the stent 100, which is rotatedand translated to ensure that the abluminal surface of the stent 100 isfully and uniformly coated. Charge from the ionized spray 230 does notbuild up on the stent 100. In an embodiment of the invention, the stentmandrel fixture 240 can be coupled to the power source 245, which willtherefore pull any charge from the stent 100 thereby leaving the stent100 neutrally charged or having a charge opposite of the ionized spray230 (e.g., having a positive charge). If a charge having the samepolarity as the ionized spray 230 were to build up on the stent 100, thestent 100 would repel the ionized spray 230, thereby preventing coatingof any surface of the stent 100. In contrast, the insulating sleeve 235does build a charge having the same polarity as the ionized spray 230.Accordingly, the insulating sleeve 235 will repel the atomized ionizedspray 230 from the interior diameter of the stent 100. The wrap aroundeffect is therefore reduced or eliminated.

The components of the coating substance or composition can include asolvent or a solvent system comprising multiple solvents; a polymer or acombination of polymers; and/or a therapeutic substance or a drug or acombination of drugs. Representative examples of polymers that can beused to coat a stent or medical device include ethylene vinyl alcoholcopolymer (commonly known by the generic name EVOH or by the trade nameEVAL); poly(hydroxyvalerate); poly(L-lactic acid); polycaprolactone;poly(lactide-co-glycolide); poly(glycerol-sebacate);poly(hydroxybutyrate); poly(hydroxybutyrate-co-valerate); polydioxanone;polyorthoester; polyanhydride; poly(glycolic acid); poly(D,L-lacticacid); poly(glycolic acid-co-trimethylene carbonate); polyphosphoester;polyphosphoester urethane; poly(amino acids); cyanoacrylates;poly(trimethylene carbonate); poly(iminocarbonate); copoly(ether esters)(e.g. PEO/PLA); polyalkylene oxalates; polyphosphazenes; biomolecules,such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronicacid; polyurethanes; silicones, polyesters; polyolefins; polyisobutyleneand ethylene-alphaolefin copolymers; acrylic polymers and copolymers;vinyl halide polymers and copolymers, such as polyvinyl chloride;polyvinyl ethers, such as polyvinyl methyl ether; polyvinylidenehalides, such as polyvinylidene fluoride and polyvinylidene chloride;polyacrylonitrile; polyvinyl ketones; polyvinyl aromatics, such aspolystyrene; polyvinyl esters, such as polyvinyl acetate; copolymers ofvinyl monomers with each other and olefins, such as ethylene-methylmethacrylate copolymers, acrylonitrilestyrene copolymers, ABS resins,and ethylene-vinyl acetate copolymers; polyamides, such as Nylon 66 andpolycaprolactam; alkyd resins; polycarbonates; polyoxymethylenes;polyimides; polyethers; epoxy resins; polyurethanes; rayon;rayon-triacetate; cellulose; cellulose acetate; cellulose butyrate;cellulose acetate butyrate; cellophane; cellulose nitrate; cellulosepropionate; cellulose ethers; and carboxymethyl cellulose.

“Solvent” is defined as a liquid substance or composition that iscompatible with the polymer and/or drug and is capable of dissolving thepolymer and/or drug at the concentration desired in the composition.Examples of solvents include, but are not limited to, dimethylsulfoxide,chloroform, acetone, water (buffered saline), xylene, methanol, ethanol,1-propanol, tetrahydrofuran, 1-butanone, dimethylformamide,dimethylacetamide, cyclohexanone, ethyl acetate, methylethylketone,propylene glycol monomethylether, isopropanol, isopropanol admixed withwater, N-methylpyrrolidinone, toluene, and mixtures and combinationsthereof. Solvents should have a high enough conductivity to enableionization of the composition if the polymer or therapeutic substance isnot conductive. For example, acetone and ethanol have sufficientconductivities of 8×10⁻⁶ and ˜10⁻⁵ siemen/m, respectively.

Examples of therapeutic substances that can be used includeantiproliferative substances such as actinomycin D, or derivatives andanalogs thereof (manufactured by Sigma-Aldrich of Milwaukee, Wis., orCOSMEGEN available from Merck). Synonyms of actinomycin D includedactinomycin, actinomycin IV, actinomycin I₁, actinomycin X₁, andactinomycin C₁. The active agent can also fall under the genus ofantineoplastic, anti-inflammatory, antiplatelet, anticoagulant,antifibrin, antithrombin, antimitotic, antibiotic, antiallergic andantioxidant substances. Examples of such antineoplastics and/orantimitotics include paclitaxel (e.g. TAXOL® by Bristol-Myers SquibbCo., Stamford, Conn.), docetaxel (e.g. Taxotere®, from Aventis S.A.,Frankfurt, Germany) methotrexate, azathioprine, vincristine,vinblastine, fluorouracil, doxorubicin hydrochloride (e.g. Adriamycin®from Pharmacia & Upjohn, Peapack N.J.), and mitomycin (e.g. Mutamycin®from Bristol-Myers Squibb Co., Stamford, Conn.). Examples of suchantiplatelets, anticoagulants, antifibrin, and antithrombins includesodium heparin, low molecular weight heparins, heparinoids, hirudin,argatroban, forskolin, vapiprost, prostacyclin and prostacyclinanalogues, dextran, D-phe-pro-arg-chloromethylketone (syntheticantithrombin), dipyridamole, glycoprotein IIb/IIIa platelet membranereceptor antagonist antibody, recombinant hirudin, and thrombininhibitors such as ANGIOMAX (Biogen, Inc., Cambridge, Mass.). Examplesof such cytostatic or antiproliferative agents include angiopeptin,angiotensin converting enzyme inhibitors such as captopril (e.g.Capoten® and Capozide® from Bristol-Myers Squibb Co., Stamford, Conn.),cilazapril or lisinopril (e.g. Prinivil® and Prinzide® from Merck & Co.,Inc., Whitehouse Station, N.J.); calcium channel blockers (such asnifedipine), colchicine, fibroblast growth factor (FGF) antagonists,fish oil (omega 3-fatty acid), histamine antagonists, lovastatin (aninhibitor of HMG-CoA reductase, a cholesterol lowering drug, brand nameMevacor® from Merck & Co., Inc., Whitehouse Station, N.J.), monoclonalantibodies (such as those specific for Platelet-Derived Growth Factor(PDGF) receptors), nitroprusside, phosphodiesterase inhibitors,prostaglandin inhibitors, suramin, serotonin blockers, steroids,thioprotease inhibitors, triazolopyrimidine (a PDGF antagonist), andnitric oxide. An example of an antiallergic agent is permirolastpotassium. Other therapeutic substances or agents which may beappropriate include alpha-interferon, genetically engineered epithelialcells, tacrolimus, dexamethasone, and rapamycin and structuralderivatives or functional analogs thereof, such as40-O-(2-hydroxy)ethyl-rapamycin (known by the trade name of EVEROLIMUSavailable from Novartis), 40-O-(3-hydroxy)propyl-rapamycin,40-O-[2-(2-hydroxy)ethoxy]ethyl-rapamycin, and 40-O-tetrazole-rapamycin.

FIG. 6 is chart illustrating spray regimes as a function of appliedvoltage and electrode separation. Applied voltage is based on the powersource 245 and the high voltage transformer 250. Electrode separationrefers to the distance between the stent 100 and the hypotube 225.Ideally, appropriate voltage is applied to the hypotube 225 to enter thefine spray regime, which provides adequate atomization. If inadequatevoltage is applied to the hypotube 225, there will not be sufficientatomization, thereby causing the composition to exit the hypotube 225 asa drip instead of as the atomized spray 230. Too much voltage on theother hand will lead to a sputtering regime in which the compositionexits the hypotube 225 in spurts instead of as the atomized spray 230.

FIG. 7 is a diagram illustrating a magnified cross section of a portionof the electrostatic spray coating system 200. Electric field lines 400extend downward from the hypotube 225 towards struts S of the stent 100.In addition, the electric field lines 400 wrap around the struts S andcan extend towards the insulating sleeve 235. During a spray coatingprocess, initially, the spray 230 follows the electric field lines 400coating the insulating sleeve 235, the abluminal surface of the stent100, and slightly coating the luminal surface of the stent 100.

During the initial stages of the spray coating process, a charge of thesame polarity as the spray 230 builds up on the insulating sleeve 235,thereby generating electric field lines 410 that counteract the electricfield lines 400. The electric field lines 410 extend outward from theinsulating sleeve 235 and prevent and repel the spray 230 from furthercoating the luminal surface of the stent 100. Accordingly, coating ofthe luminal surface of the stent 100 is substantially limited. In anembodiment of the invention, the insulating sleeve 235 (or coating ornonconductive mandrel parts) can come pre-charged so that the electricfield lines 410 are generated prior to the beginning of the spraycoating process, thereby preventing or reducing any coating of theluminal surface of the stent 100 with the spray 230. The insulatingsleeve 235 (or coating or nonconductive mandrel parts) can bepre-charged by, for example, spraying the sleeve 235 or coating beforeloading the stent 100 onto the system 200.

In some embodiments, during the spray coating process, the stent 100 isin electrical contact with the stent mandrel fixture 240 and is groundedand can be supplied with a charge opposite the charge of the spray 230.The fixture 240 can be supplied with an opposite charge by electricallycoupling the fixture 240 to the power source 245, which supplies a firstcharge to the spray 230 and an opposite charge to the fixture 240. Thisdifference in polarity increases the attraction of the spray 230 to thestent 100, therefore increasing coating of the stent 100. Thisdifference in polarity can also compensate for misalignment of thehypotube 225 with the stent 100, which is a critical issue during theconventional applications of a composition to a drug releasing stent.Specifically, the polarity difference will pull the spray 230 towardsthe stent 100 even if the stent 100 is not positioned directly beneaththe hypotube 225.

FIG. 5 is a flowchart illustrating a method 500 of electrostatic spraycoating. In an embodiment of the invention, the system 200 executes themethod 500. First, a high voltage is applied (510) to the composition asit travels through the hypotube 225, which ionizes and atomizes thecomposition so that it can be ejected as atomized ionized spray 230. Thestent 100 is then grounded and optionally charged (520) so that there isa charge or polarity differential between the spray 230 and the stent100 so that the stent 100 attracts the spray 230. The ionized atomizedcomposition is then sprayed (530) onto the stent 100 and the insulatingsleeve 235, thereby forming a charge on the sleeve 235 so that anyadditional spray 230 is repelled from the sleeve 235 and therefore theluminal surface of the stent 100. In an alternative embodiment, thesleeve 235 may be pre-charged (e.g., by spraying the sleeve 235 beforeloading the stent 100 onto the mandrel fixture) and so the spraying(530) only covers the abluminal surface of the stent 100. The stent 100is also rotated and translated (540) during the spraying (530) so thatthe entire abluminal surface of the stent 100 can be coated. The method500 then ends.

It will be appreciated by one of ordinary skill in the art that theapplying (510), grounding/charging (520), spraying (530), androtating/translating (540) can occur in any order and is not limited tothe order disclosed above. Further, the applying (510),grounding/charging (520), spraying (530), and rotating/translating (540)can occur substantially simultaneously instead of sequentially.

The following examples are provided:

EXAMPLE 1

In an example spray process, parameters according to Table I were used.The composition comprised 2% weight Poly(DL-Lactide) in 80/20acetone/cyclohexanone solvent mixture. A blue dye was added to theformulation to replace the drug and to provide the contrast. The polymerto dye ratio was 1 to 1. The stent mandrel fixture was grounded througha wire with an alligator clip. The spray cycle was programmed for10-seconds spray and 10-seconds drying cycle. The total coating weightwas 630 μg. The stent was an 18 mm Vision stent available from Guidant.A pellethane tube was used between the stent and the mandrel to createthe repulsive force.

TABLE I Mandrel Spin Rate 150 RPM Start Position  25 mm End Position  54mm Dry Position 120 mm Drying Nozzle  60° C. Temperature Set PointDrying Air Pressure  23 PSI Electrode Separation  20-25 mm AppliedVoltage  4-10 KV

EXAMPLE 2

Cleaned 18 mm Vision stents (Guidant Corp.) were first primered with 2wt % of poly(butyl methacrylate) solution using a modified N1537 spraycoater and the coating weight was in the range of 75 to 90 μg. Theprimered stents were mounted on a metallic mandrel with one additionalmetallic collet, which was used to support the stent and to provide thecontact points for ground. The mandrel was grounded through a wire withan alligator clip. SOLEF formulation was used for the drug coat ofeverolimus (190 μg dose with polymer to drug ration of 3.12 to 1). SOLEFis a trade name of poly(vinylidene fluoride-co-hexafluoropropene)available from Solvay Fluoropolymers, Inc. of Houston, Tex. Thedeposited rate per spray cycle was controlled in the range of 15 to 20μg. The spray cycle was programmed for 10-seconds spray and 10-secondsdry cycle. The drying temperature was set at 60 deg. C. and the airpressure was set at 23 psi. A post oven bake at 50 deg. C. for 1 hourwas conducted and the coating weight on the 18 mm Vision stents weretargeted in the range of 785 μg to 835 μg.

EXAMPLE 3

A bare metal stent was mounted over a metallic spray mandrel. Themandrel was grounded through a wire with an alligator clip. D, L-PLAformulation, 80/20 acetone to cyclohexanone with 1:1 polymer toeverolimus ratio, was used for the drug coating. The deposit rate perspray cycle was controlled in the range of 70 to 80 μg. The voltage wascontrolled between 6 to 8 KV. The spray cycle was programmed for10-seconds spray and 10-seconds dry cycle. The dry temperature was setat 60 deg. C. and the air pressure was set at 23 psi. A post bake at 50deg. C. for 1 hour was conducted and the coating weight on the 18 mmstent was targeted in range of 630 μg to 730 μg.

While particular embodiments of the present invention have been shownand described, it will be obvious to one of ordinary skill in the artthat changes and modifications can be made without departing from thisinvention in its broader aspects. For example, after application of thecoating to the abluminal surface of the stent 100 as described above,the luminal surface of the stent 100 can be coated with a differentcoating via spray coating, electroplating or other technique. Therefore,the appended claims are to encompass within their scope all such changesand modifications as fall within the true spirit and scope of thisinvention.

1. A stent mandrel fixture to support a stent during application of acharged coating substance to the stent, comprising: a first mandrelcomponent in conductive contact with the stent, the first mandrelcomponent including a support element to contact one end of the stentand a lock element to contact the other end of the stent; and a secondmandrel component connecting the support element to the lock element andpositioned at least partially within a bore of the stent, the secondmandrel component being spaced from the walls of the bore, having anoutermost surface exposed to the walls of the bore, and being made froma nonconductive material or being coated with a nonconductive material.2. The fixture of claim 1, wherein the first mandrel component providesa charge differential to the stent relative to the coating substance. 3.The fixture of claim 1, wherein the first mandrel component iselectrically coupled to a power source to provide a charge to the stenthaving a polarity opposite of the coating substance.
 4. The fixture ofclaim 1, wherein the nonconductive material or the sleeve is capable ofcollecting a charge of the same polarity as the coating substance. 5.The fixture of claim 1, wherein the coating substance includes aconductive solvent.
 6. The fixture of claim 1, wherein the supportelement or the lock element is made from a non-conductive material, iscoated with a non-conductive material or has a non-conductive sleevedisposed thereon.
 7. The fixture of claim 1, wherein the support elementincludes a coning end portion that penetrates into an end of the stent.8. The fixture of claim 7, wherein a tip of the coning end portion ismade from a non-conductive material, is coated with a non-conductivematerial or has a non-conductive sleeve disposed thereon.
 9. The fixtureof claim 1, wherein the lock element includes a coning end portion thatpenetrates into an end of the stent.
 10. The fixture of claim 9, whereina tip of the coning end portion is made from a non-conductive material,is coated with a non-conductive material or has a non-conductive sleevedisposed thereon.
 11. The fixture of claim 1, wherein the first mandrelcomponent is in contact with an end segment of the stent.
 12. Thefixture of claim 1, wherein the second mandrel component has an outersurface and the outer surface does not abut a surface of the bore.
 13. Afixture to support a stent during application of a charged substance tothe stent, comprising: a mandrel arm including a first section having afirst diameter, the first section being in conductive contact with anend of the stent; and a second section having a second diameter, lessthan the first diameter, disposed coaxially with the first section,extending at least partially through a longitudinal bore of the stentand spaced from the walls of the bore, the second section beingconfigured to receive the charged substance on its outermost surface soas to minimize or eliminate the wrap around effect of the chargedsubstance around stent struts to prevent any substance from beingapplied to an inner side of the stent or to reduce the amount ofsubstance applied to an inner side of the stent.
 14. The fixture ofclaim 13, wherein the second section includes an element configured tocollect charged particles applied to the element.
 15. The fixture ofclaim 13, wherein the second section includes an element configured torepel charged particles applied to the element.
 16. The fixture of claim15, wherein the element is the charged substance.
 17. The fixture ofclaim 13, wherein the first section is for applying a charge to thestent and/or to ground the stent.
 18. The fixture of claim 13, whereinthe mandrel arm is configured for supporting the stent as a cantilever.